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| United States Patent |
5,501,111
|
|
Sonderegger
, et al.
|
March 26, 1996
|
Force sensor systems especially for determining dynamically the axle
load, speed, wheelbase and gross weight of vehicles
Abstract
The invention comprises a force measuring system and an assembly procedure
for the dynamic determination especially of axle loads, speed, as well as
for assembly of force and acceleration sensors. The force measuring system
can be laid permanently in the roadway. It is of modular assembly,
consisting of the amplifier, sensor and terminal module. The sensor module
consists of a hollow section, in which piezo-elements are fitted under
elastic preload. The hollow section is designed so that it can be opened
elastically by lateral clamping, to allow mechanically interconnected
piezo-elements to be inserted simply. After releasing this clamping, the
piezo-elements are under high elastic preload. The same design principle
can also be applied generally to force pressure and acceleration sensors.
The elastical clamping force, vital for perfect force transmission to the
piezo-elements, is for assembly purposes easily overcome by a proper
lateral clamping force.
| Inventors:
|
Sonderegger; Hans C. (Neftenbach, CH);
Calderara; Reto (Neftenbach, CH);
Cavalloni; Claudio (Neftenbach, CH)
|
| Assignee:
|
Kistler Instrumente AG (Winterthur, CH)
|
| Appl. No.:
|
264062 |
| Filed:
|
June 22, 1994 |
Foreign Application Priority Data
| Current U.S. Class: |
73/862.642; 29/25.35; 73/DIG.4 |
| Intern'l Class: |
G01L 001/04 |
| Field of Search: |
73/862.625,862.642,862.68,DIG. 4,146.2,146.3,146.4,146.5
29/25.35
|
References Cited
U.S. Patent Documents
| 3239696 | Mar., 1966 | Burkhalter et al. | 73/DIG.
|
| 3614488 | Oct., 1971 | Sonderegger et al. | 73/DIG.
|
| 4059999 | Nov., 1977 | Engeler et al. | 73/745.
|
| 4846861 | Aug., 1987 | Morii | 73/DIG.
|
| 5072611 | Dec., 1991 | Budd et al. | 73/118.
|
| 5214967 | Jun., 1993 | Grogan | 73/862.
|
| 5329823 | Jul., 1994 | Sonderregger et al. | 73/862.
|
| 5337461 | Aug., 1994 | Falcus | 29/25.
|
| 5345428 | Sep., 1994 | Arnold et al. | 29/25.
|
Primary Examiner: Chilcot; Richard
Assistant Examiner: Dougherty; Elizabeth L.
Attorney, Agent or Firm: Barnes & Thornburg
Parent Case Text
CROSS-REFERENCE
This is a continuation-in-part of Ser. No. 08/114,700, filed Aug. 31,
1993now abandoned, which is a continuation of 07/995,831, filed Dec. 23,
1992, now U.S. Pat. No. 5,265,481, which is a continuation of 07/810,039,
filed Dec. 19, 1991, now abandoned.
Claims
We claim:
1. Force sensor system for dynamically determining vehicle axle load,
speed, wheelbase and gross weight, comprising:
an elastically deformable sleeve having a longitudinal recess;
a number of piezo-elements located within the recess for obtaining a
constant force measuring sensitivity over the whole length of the sleeve;
said piezo-elements being press-fitted by force-introducing parts in said
recess between said sleeve and said piezo-elements so that the
piezo-elements are under permanent elastic preload along a first axis
transverse a longitudinal axis of said recess by elastic deformation of
said sleeve.
2. The force sensor system according to claim 1, wherein the sleeve is a
tube of circular cross section in which piezo-discs are fitted between
force-introducing elements in said recess so that the piezo-discs are
under permanent elastic preload.
3. The force sensor system according to claim 1, wherein the sleeve is a
tube of non-circular cross section in which piezo-disks are fitted between
force-introducing parts in said recess so that the piezo-discs are under
permanent elastic preload.
4. A force acceleration sensor comprising:
a center post;
a pair of piezo discs separated by said post;
a pair of force-introducing parts separated by said piezo discs and said
post; and
an elastically deformable sleeve encompassing said post, piezo discs and
force-introducing parts and elastically preloading said piezo discs by
elastical deformation of said sleeve.
5. A sensor according to claim 4 wherein said post is fixed in and extends
along a first axis of a sensor case and said sleeve preloads said piezo
discs along a second axis transverse to said first axis.
6. A sensor system according to claim 5, wherein the piezo-discs are of
rectangular shape and are sensitive in shear mode along said first axis.
7. A sensor according to claim 4, wherein between a signal contact and a
signal cable an impedance converter/amplifier device is interconnected.
8. A sensor according to claim 4, wherein the piezo-discs are made of piezo
ceramic.
9. A sensor according to claim 4, wherein the piezo-discs are made of a
single crystal material.
10. A sensor according to claim 4 wherein said sleeve is an elastic metal.
11. A sensor according to claim 4 wherein said post extends along a first
axis of said sleeve which forms a sensor case and said sleeve preloads
said piezo discs along a second axis transverse to said first axis.
12. A method for assembling and preloading a force sensor comprising:
distorting a cross-sectional area of a sleeve to enlarge one
cross-sectional length of a longitudinal recess in the sleeve;
inserting into the recess a piezo assembly having a pair of piezo discs
separated by a center post and between a pair of force-introducing parts;
relaxing the distortion whereby the piezo discs are preloaded elastically
so that any air gaps between the force-introducing parts, piezo discs and
center post are removed.
13. A force sensor comprising:
a sensor case;
a center post unitary with said sensor case;
a pair of piezo discs separated by said post;
a pair of force-introducing parts separated by said piezo discs and said
post; and
an elastically deformable sleeve encompassing said post, piezo discs and
force-introducing parts and elastically preloading said piezo discs.
14. A force sensor comprising:
a center post;
a pair of piezo discs separated by said post;
a pair of force-introducing parts separated by said piezo discs and said
post; and
an elastically deformable sleeve unitary with said force-introducing parts
and encompassing said post, piezo discs and force-introducing parts and
elastically preloading said piezo discs.
Description
BACKGROUND AND SUMMARY OF THE INVENTION
The invention relates to a force sensor system, especially for determining
dynamically the axle load and speed, and to a circular clamp arrangement
that allows simple assembly of force, pressure and acceleration sensors.
Familiar is the static weight measuring of vehicles by means of
weighbridges, or with weighing devices placeable on the roadway surface
and generally easily transportable. One example of these is Swiss Patent
CH 597 595, which describes a weighing platform having a weighing plate
resting on a base plate through hydraulic transducers. The pressure of the
liquid compressed by the loading is measured. Weighing platforms operating
with strain gauge elements are known also. Thus, the weighing platform
described in U.S. Pat. No. 3,949,822consists of a base plate flexible
under load and a weighing plate, whereby the flexure is measured by means
of strain gauges. Both weighing platforms are relatively thick and
provided with ramps for driving up. Obviously they are suited only for
static weighing of axle loads, or with the vehicle moving over very
slowly.
In view of the growing vehicular traffic, however, a modern axle load
determination must be capable of being performed dynamically, i.e.,
without the vehicle having to reduce its speed. Already attempts have been
made to determine axle load and vehicle weight by means of so-called
piezo-cables let into the top surfacing of a roadway. As signal elements,
piezo-cables contain piezo-foils or piezo-compound. The measuring
sensitivity of these depends very much on temperature.
A measuring arrangement with a piezo-cable laid in the top surfacing is
prone to instability owing to the small force-absorbing surface and the
compressibility of a plastic cable, which causes varying measuring
sensitivity and other measuring inadequacies, such as instability of the
measuring zero. Moreover, the measuring results of the axle load and
vehicle weight are too inaccurate, mostly due to the often unsatisfactory
insulation resistance of piezo-cables. The object of the present invention
is to provide a force measuring system which overcomes the disadvantages
named and enables dynamic determination of the axle load, speed, wheelbase
and gross weight, employing advantageously two force sensor systems
successively spaced at a short interval. In addition the invention shows a
circular clamp system that can be used in other sensor systems.
The force sensor system must be suitable for any width of roadway and
installable in simple fashion, for asphalt as for concrete roads.
The problem is solved by laying in the roadway a force sensor consisting of
a hollow section in which a number of piezo-elements are fitted, giving a
constant force measuring sensitivity over the entire length of the hollow
section and arranged to allow electrical connection of the piezo-elements
in series, group wise or individually, and so that simply layable modules
result, consisting of amplifier, sensor and termination units.
The same hollow section is used generally on sensors for force, pressure
and acceleration equipped with piezo-elements utilizing the shear effect.
Other objects, advantages and novel features of the present invention will
become apparent from the following detailed description of the invention
when considered in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross section through a force sensor according to the invention
with cylindrical hollow section (tubular sensor).
FIG. 2 is an installation procedure by opening a hollow section of circular
cross section by lateral clamping.
FIG. 3a is an amplifier module connected straight to the force sensor.
FIG. 3b is an amplifier module connected separately by corrugated tubing,
linked with force sensor.
FIG. 4 is an amplifier module.
FIG. 5 is a sensor module with plug-and-socket connection.
FIG. 6 is a cross section through a tubular hollow profile with
force-introducing parts insulated electrically from the tube.
FIG. 7 is a cross section through a hexagonal hollow section.
FIG. 8 is a cross section through an acceleration measuring sensor.
FIG. 9 is a cross section through a tubular hollow profile of FIG. 8.
FIG. 10 is a variant of a cross section of FIG. 8.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a perspective view with cross section through a force sensor
according to the invention with cylindrical hollow section (tubular
sensor). The hollow section 1 contains a recess 2 in its longitudinal
axis, into which a piezo-element 4 is press-fitted. A Piezo-element 4
consists of the force-introducing parts 7 and two piezo-discs 3. Between
them is an electrically conductive foil 5 as signal carrier. The resulting
piezo-element 4 is designed to be sensitive essentially to a force acting
in the Z direction. This force P is introduced via a force-transmission
section 6. As is explained later, the force sensor lies generally in the
top surfacing of a roadway and the forces P originate from the weight of a
vehicle passing over it. In the electrically conductive foil 5 they
generate positive electrical charges for example, in the force-introducing
parts 7 negative charges. The force-introducing parts 7 may take the shape
of insulating ceramic or plastic strips passing through the entire hollow
section, or individual parts.
However, it is also possible to make the force-introducing parts 7 of
metal, and lead off the negative charges directly via a metal hollow
section, as will be shown in FIG. 6.
The piezo-element 4 is mechanically preloaded. This preloading is obtained
by pressing the hollow section 1 together laterally in the X direction
during assembly, with pressing clamps, so that the recess 2 widens in the
Z direction by the amount .DELTA.h (FIG. 2), after which the piezo-element
4 is guided into the recess 2. After removing the lateral pressure the
widening disappears also, and with suitable dimensioning of the hollow
section 1 and the recess 2 the piezo-element sustains an elastic preload
(FIG. 2). As materials for the piezo-discs 3 both crystalline materials
and piezoceramics or piezo-foils may be employed. The hollow section 1 may
for example consist of a stainless steel tube with drawn inside profile,
or of an extruded tube of aluminum alloy. It might just as well consist of
a suitable plastic. An outside diameter of about 20 to 40 mm has proved
efficacious for installation purpose. Also, the outside form of the hollow
section 1 could be different from circular, for instance with flange
sections.
FIG. 3a shows how the amplifier module 11 is attached directly to the force
sensor of length L. FIG. 3b shows this connection in the form of a
corrugated tubing cable 10. The force sensor of length L (which may be up
to 20 m) is divided into separate pieces of length L.sub.1 for
manufacturing and installation reasons. These sensor modules 12 are
preferably about 1 to 2 m long and can be plugged together, welded or
glued. To match the force sensor to the roadway geometry it is advisable
to provide the coupling elements with a certain elasticity, so that angle
.gamma. is about 1.degree. to 3.degree.. The last module is closed with
end piece 13.
FIG. 4 shows a tubular-shaped amplifier module 11 with coupling piece 14
direct mounted.
FIG. 5 shows a sensor module 12 with positive and negative coupling pieces
14.
FIG. 6 shows a cross section through an embodiment of the force sensor
according to the invention similar to that in FIG. 1. The principal
difference from the embodiment in FIG. 1 is the commercially obtainable
tube, in that the force is introduced through separate force-introducing
parts 7, i.e., isolated from the hollow section 1 by an insulating layer
9. This makes it possible to keep both signal leads from the hollow
section 1 separate, so that the measuring system is less susceptible to
interference. The signals are led out on the one hand by the conductive
foil 5, and on the other hand by the signal conductors 8, which are laid
electrically conductive in grooves of the force-introducing parts 7.
Mechanically to the embodiment of the force sensor shown in FIG. 2
constitutes a variant of FIG. 1, in that the hollow section 1 consists of
a tube and can likewise be deformed elastically by lateral clamping in a
special vice.
FIG. 7 shows a hollow section according to the invention, in the form of a
hexagonal tube which can likewise be opened mechanically by clamping in
the X direction. Other hollow sections are also conceivable.
FIG. 8 shows a cross-section of an accelerometer with similar tubular
section for clamping and opening by lateral force of piezo-electric
elements.
FIG. 9 shows the cross-section A--A through the acceleration sensor of FIG.
8. The tubular section 20 acts similar as in FIG. 6. The lateral force F
opens the hollow section 29 in order to introduce the assembly consisting
of force transmission post 22, piezo-discs 24 and clamp force transmission
parts 21. After release of the lateral force F, the piezo assembly is in a
solid compression position transmitting force P via force transmission
post 22 to the piezo-discs 24 and to the clamp force transmission parts 21
which act as seismic masses. For measuring acceleration in the Z
direction, the piezo-discs 24 must be sensitive in shear direction. For
acceleration in X direction, they must be sensitive in compression. The
signal contact 30 is conducted to the signal cable 25. It is state of the
art to introduce an amplifier module into the sensor housing. After
assembly, the sensor is closed by cap 31.
FIG. 10 shows the cross-section of a variant to FIG. 9 where the clamping
ring section 26 and the clamping force transmission part 27 are made of
one piece, for example, by injection molding whereby both parts together
act as seismic mass. In order to increase the signal size it is possible
to use a metal heavier than steel.
The invention describes a new system of piezo-electric sensors utilized for
force measurements of automobile axles on road surfaces or for general
force and acceleration sensors. The main principle is the requirement that
in order to receive a solid state system no airgaps are allowed between
force transmitting members and piezo-discs. The system thus must be set on
a mechanical pretension. This is especially important for a utilization in
accelerometers (FIG. 8) where the clamping force must be high enough to
avoid any slippage when the instrument is accidentally dropped.
The system of a mechanical opening of the hollow sections 1, 29 by a
lateral clamping force F allows a very simple and reliable assembly method
for piezo-electric sensing systems allowing measurement of forces,
pressures and accelerations.
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